RFC 6560

One-Time Password (OTP) Pre-Authentication

Internet Engineering Task Force (IETF) G. Richards
Request for Comments: 6560 RSA, The Security Division of EMC
Category: Standards Track April 2012
ISSN: 2070-1721
One-Time Password (OTP) Pre-Authentication
Abstract
The Kerberos protocol provides a framework authenticating a client
using the exchange of pre-authentication data. This document
describes the use of this framework to carry out One-Time Password
(OTP) authentication.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6560.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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1. Introduction
1.1. Scope
This document describes a Flexible Authentication Secure Tunneling
(FAST) [RFC6113] factor that allows One-Time Password (OTP) values to
be used in the Kerberos V5 [RFC4120] pre-authentication in a manner
that does not require use of the user's Kerberos password. The
system is designed to work with different types of OTP algorithms
such as time-based OTPs [RFC2808], counter-based tokens [RFC4226] and
challenge-response systems such as [RFC2289]. It is also designed to
work with tokens that are electronically connected to the user's
computer via means such as a USB interface.
This FAST factor provides the following facilities (as defined in
[RFC6113]): client-authentication, replacing-reply-key, and KDC-
authentication. It does not provide the strengthening-reply-key
facility.
This proposal is partially based upon previous work on integrating
single-use authentication mechanisms into Kerberos [HORENEZ004].
1.2. Overall Design
This proposal supports four- and two-pass variants. In the four-pass
system, the client sends the Key Distribution Center (KDC) an initial
AS-REQ, and the KDC responds with a KRB-ERROR containing pre-
authentication data that includes a random nonce. The client then
encrypts the nonce and returns it to the KDC in a second AS-REQ.
Finally, the KDC returns the AS-REP. In the two-pass variant, the
client encrypts a timestamp rather than a nonce from the KDC, and the
encrypted data is sent to the KDC in the initial AS-REQ. The two-
pass system can be used in cases where the client can determine in
advance that OTP pre-authentication is supported by the KDC, which
OTP key should be used and the encryption parameters required by the
KDC.
In both systems, in order to create the message sent to the KDC, the
client must generate the OTP value and two keys: the classic Reply
Key used to decrypt the KDC's reply and a key to encrypt the data
sent to the KDC. In most cases, the OTP value will be used in the
key generation, but in order to support algorithms where the KDC
cannot obtain the value (e.g., [RFC2289]), the system supports the
option of including the OTP value in the request along with the
encrypted nonce. In addition, in order to support situations where
the KDC is unable to obtain the plaintext OTP value, the system also
supports the use of hashed OTP values in the key derivation.

The pre-authentication data sent from the client to the KDC is sent
within the encrypted data provided by the FAST pre-authentication
data type of the AS-REQ. The KDC then obtains the OTP value,
generates the same keys, and verifies the pre-authentication data by
decrypting the nonce. If the verification succeeds, then it confirms
knowledge of the Reply Key by using it to encrypt data in the AS-REP.
1.3. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document assumes familiarity with the Kerberos
pre-authentication framework [RFC6113] and so freely uses terminology
and notation from that document.
The word padata is used as shorthand for pre-authentication data.
2. Usage Overview
2.1. OTP Mechanism Support
As described above, this document describes a generic system for
supporting different OTP mechanisms in Kerberos pre-authentication.
To ensure interoperability, all implementations of this specification
SHOULD provide a mechanism (e.g., a provider interface) to add or
remove support for a particular OTP mechanism.
2.2. Pre-Authentication
The approach uses pre-authentication data in AS-REQ, AS-REP, and
KRB-ERROR messages.
In the four-pass system, the client begins by sending an initial
AS-REQ to the KDC that may contain pre-authentication data such as
the standard Kerberos password data. The KDC will then determine, in
an implementation dependent fashion, whether OTP authentication is
required and if it is, it will respond with a KRB-ERROR message
containing a PA-OTP-CHALLENGE (see Section 4.1) in the PA-DATA.
The PA-OTP-CHALLENGE will contain a KDC-generated nonce, a list of
hash algorithm identifiers, and an iteration count if hashed OTP
values are used (see Section 3.6) and OPTIONAL information on how the
OTP should be generated by the client. The client will then generate
the OTP value and two keys: a Client Key to encrypt the KDC's nonce
and a Reply Key used to decrypt the KDC's reply.

As described in Section 5.4.1 of [RFC6113], the FAST system uses an
Armor Key to set up an encrypted tunnel for use by FAST factors. As
described in Section 3.6 of this document, the Client Key and Reply
Key will be generated from the Armor Key and the OTP value, unless
the OTP algorithm does not allow the KDC to obtain the OTP value. If
hash algorithm identifiers were included in the PA-OTP-CHALLENGE,
then the client will use the hash of the OTP value rather than the
plaintext value in the key generation. Both keys will have the same
encryption type as the Armor Key.
The generated Client Key will be used to encrypt the nonce received
from the KDC. The encrypted value along with optional information on
how the OTP was generated are then sent to the KDC in a
PA-OTP-REQUEST (see Section 4.2) encrypted within the armored-data of
a PA-FX-FAST-REQUEST PA-DATA element of a second AS-REQ.
In the two-pass system, the client sends the PA-OTP-REQUEST in the
initial AS-REQ instead of sending it in response to a
PA-OTP-CHALLENGE returned by the KDC. Since no challenge is received
from the KDC, the client includes an encrypted timestamp in the
request rather than the encrypted KDC nonce.
In both cases, on receipt of a PA-OTP-REQUEST, the KDC generates the
keys in the same way as the client, and uses the generated Client Key
to verify the pre-authentication by decrypting the encrypted data
sent by the client (either nonce or timestamp). If the validation
succeeds, then the KDC will authenticate itself to the client and
confirm that the Reply Key has been updated by using the generated
Reply Key in the AS-REP response.
2.3. PIN Change
Most OTP tokens involve the use of a Personal Identification Number
(PIN) in the generation of the OTP value. This PIN value will be
combined with the value generated by the token to produce the final
OTP value that will be used in this protocol.
If, following successful validation of a PA-OTP-REQUEST in an AS-REQ,
the KDC determines that the user's PIN has expired and needs to
change, then it SHOULD respond with a KRB-ERROR of type
KDC_ERR_PIN_EXPIRED. It MAY include formatting information on the
PIN in a PA-OTP-PIN-CHANGE (see Section 4.3) encrypted within the
armored-data of the PA-FX-FAST-REPLY PA-DATA element.
KDC_ERR_PIN_EXPIRED 96

If the PIN change is to be handled by a PIN-change service, then it
is assumed that authentication to that service will succeed if the
PIN has expired.
If the user's PIN has not expired but has been changed, then the KDC
MAY return the new value to the client in a PA-OTP-PIN-CHANGE
encrypted within the armored-data of the PA-FX-FAST-REPLY PA-DATA
element of the AS-REP. Similarly, if a PIN change is not required,
then the KDC MAY return a PA-OTP-PIN-CHANGE to inform the client of
the current PIN's expiration time.
2.4. Resynchronization
It is possible with time- and event-based tokens that the OTP server
will lose synchronization with the current token state. For example,
event-based tokens may drift since the counter on the token is
incremented every time the token is used, but the counter on the
server is only incremented on an authentication. Similarly, the
clocks on time-based tokens may drift.
Methods to recover from this type of situation are OTP
algorithm-specific but may involve the client sending a sequence of
OTP values to allow the server to further validate the correct
position in its search window (see Section 7.4 of [RFC4226] for an
example).
If, when processing a PA-OTP-REQUEST, the pre-authentication
validation fails for this reason, then the KDC MAY return a KRB-ERROR
message. The KRB-ERROR message MAY contain a PA-OTP-CHALLENGE in the
PA-DATA with a single otp-tokenInfo representing the token used in
the initial authentication attempt but with the "nextOTP" flag set.
If this flag is set, then the client SHOULD re-try the authentication
using an OTP value generated using the token in the "state" after
that used in the failed authentication attempt, for example, using
the next time interval or counter value.
3. Pre-Authentication Protocol Details
3.1. Initial Client Request
In the four-pass mode, the client begins by sending an initial
AS-REQ, possibly containing other pre-authentication data. If the
KDC determines that OTP-based pre-authentication is required and the
request does not contain a PA-OTP-REQUEST, then it will respond as
described in Section 3.2.

If the client has all the necessary information, it MAY use the
two-pass system by constructing a PA-OTP-REQUEST as described in
Section 3.3 and including it in the initial request.
3.2. KDC Challenge
If the user is required to authenticate using an OTP, then the KDC
SHALL respond to the initial AS-REQ with a KRB-ERROR (as described in
Section 2.2 of [RFC6113]), with a PA-OTP-CHALLENGE contained within
the enc-fast-rep of the armored-data of a PA-FX-FAST-REPLY encrypted
under the current Armor Key as described in [RFC6113].
If the OTP mechanism is to be carried out as an individual mechanism,
then the PA-OTP-CHALLENGE SHALL be carried within the padata of the
KrbFastResponse. Alternatively, if the OTP mechanism is required as
part of an authentication set, then the PA-OTP-CHALLENGE SHALL be
carried within a PA-AUTHENTICATION-SET-ELEM as described in Section
5.3 of [RFC6113].
The PA-OTP-CHALLENGE SHALL contain a nonce value to be returned
encrypted in the client's PA-OTP-REQUEST. This nonce string MUST
contain a randomly chosen component at least as long as the Armor Key
length (see [RFC4086] for an in-depth discussion of randomness). In
order to allow it to maintain any state necessary to verify the
returned nonce, the KDC SHOULD use the mechanism described in Section
5.2 of [RFC6113].
The KDC MAY use the otp-service field to assist the client in
locating the OTP token to be used by identifying the purpose of the
authentication. For example, the otp-service field could assist a
user in identifying the token to be used when a user has multiple OTP
tokens that are used for different purposes. If the token is a
connected device, then these values SHOULD be an exact octet-level
match for the values present on the target token.
The KDC SHALL include a sequence of one or more otp-tokenInfo
elements containing information on the token or tokens that the user
can use for the authentication and how the OTP value is to be
generated using those tokens. If a single otp-tokenInfo element is
included, then only a single token is acceptable by the KDC, and any
OTP value generated by the client MUST be generated according to the
information contained within that element. If more than one
otp-tokenInfo element is included, then the OTP value MUST be
generated according to the information contained within one of those
elements.

The KDC MAY include the otp-vendor field in an otp-tokenInfo to
identify the vendor of the token that can be used in the
authentication request in order to assist the client in locating that
token.
If the KDC is able to obtain the OTP values for the token, then the
OTP value SHOULD be used in the key generation as described in
Section 3.6; therefore, the KDC SHOULD set the "must-encrypt-nonce"
flag in the otp-tokenInfo. If the KDC is unable to obtain the OTP
values for the token, then the "must-encrypt-nonce" flag MUST NOT be
set. If the flag is not set, then the OTP value will be returned by
the client in the otp-value field of the PA-OTP-REQUEST and so, if
returning of OTP values in this way does not conform to KDC policy,
then the KDC SHOULD NOT include the otp-tokenInfo for that token in
the PA-OTP-CHALLENGE.
If the KDC requires that hashed OTPs be used in the key generation as
described in Section 3.6 (for example, it is only able to obtain
hashed OTP values for the token), then it MUST include the supported
hash algorithms in order of preference in the supportedHashAlg of the
otp-KeyInfo and the minimum value of the iteration count in the
iterationCount element.
Since the OTP mechanism described in this document is replacing the
Reply Key, the classic shared-key system cannot be relied upon to
allow the client to verify the KDC. Therefore, as described in
Section 3.4 of [RFC6113], some other mechanism must be provided to
support this. If the OTP value is used in the Reply Key generation,
then the client and KDC have a shared key and KDC-authentication is
provided by the KDC using the Reply Key generated from the OTP value.
However, if the OTP value is sent in the otp-value element of the
PA-OTP-REQUEST, then there is no such shared key and the OTP
mechanism does not provide KDC-authentication. Therefore, if the OTP
mechanism is not being used in an environment where
KDC-authentication is being provided by other means (e.g., by the use
of a host-key-based Armor Key), then the KDC MUST NOT include any
otp-tokenInfo elements in the PA-OTP-CHALLENGE that do not have the
"must-encrypt-nonce" flag set.
If the OTP for a token is to be generated using a server-generated
challenge, then the value of the challenge SHALL be included in the
otp-challenge field of the otp-tokenInfo for that token. If the
token is a connected device and the OTP is to be generated by
combining the challenge with the token's current state (e.g., time),
then the "combine" flag SHALL be set within the otp-tokenInfo
containing the challenge.

If the KDC can determine which OTP token key (the seed value on the
token used to generate the OTP) is to be used, then the otp-tokenID
field MAY be included in the otp-tokenInfo to pass that value to the
client.
The otp-algID field MAY be included in an otp-tokenInfo to identify
the algorithm that should be used in the OTP calculation for that
token. For example, it could be used when a user has been issued
with multiple tokens that support different algorithms.
If the KDC can determine that an OTP token that can be used by the
user does not require the client to collect a PIN, then it SHOULD set
the "do-not-collect-pin" flag in the otp-tokenInfo representing that
token. If the KDC can determine that the token requires the client
to collect a PIN, then it SHOULD set the "collect-pin" flag. If the
KDC is unable to determine whether or not the client should collect a
PIN, then the "collect-pin" and "do-not-collect-pin" flags MUST NOT
be set.
If the KDC requires the PIN of an OTP token to be returned to it
separately, then it SHOULD set the "separate-pin-required" flag in
the otp-KeyInfo representing that token.
If the KDC requires that the OTPs generated by the token have a Luhn
check digit appended, as defined in [ISOIEC7812], then it MUST set
the "check-digit" flag. This flag only applies if the format of the
OTP is decimal; therefore, the otp-format field, if present, MUST
have the value of "decimal".
Finally, in order to support connected tokens that can generate OTP
values of varying lengths or formats, the KDC MAY include the desired
otp-length and format of the OTP in the otp-length and otp-format
fields of an otp-tokenInfo.
3.3. Client Response
The client response SHALL be sent to the KDC as a PA-OTP-REQUEST
included within the enc-fast-req of the armored-data within a
PA-FX-FAST-REQUEST encrypted under the current Armor Key as described
in [RFC6113].
In order to generate its response, the client MUST generate an OTP
value. If the PA-OTP-CHALLENGE contained one or more otp-tokenInfo
elements, then the OTP value MUST be based on the information
contained within one of those elements.

The otp-service, otp-vendor, otp-tokenID, otp-length, otp-format, and
otp-algID elements of the PA-OTP-CHALLENGE are provided by the KDC to
assist the client in locating the correct token to use, but the use
of the above fields will depend on the type of token.
If the token is a disconnected device, then the values of otp-service
and otp-vendor MAY be displayed to the user in order to help the user
select the correct token, and the values of otp-algID, otp-tokenID,
otp-length, and otp-format MAY be ignored.
If the token is a connected device, then these values, if present,
SHOULD be used by the client to locate the correct token. When the
token is connected, clients MUST support matching based on a binary
comparison of the otp-vendor and otp-service strings when comparing
the values against those present on the token. Clients MAY have
other comparisons including normalization insensitive comparisons to
try and find the right token. The values of otp-vendor and
otp-service MAY be displayed to prompt the user if the correct token
is not found.
If the "nextOTP" flag is set in the otp-tokenInfo from the
PA-OTP-CHALLENGE, then the OTP value MUST be generated from the next
token state rather than that used in the previous PA-OTP-REQUEST for
that token. The "nextOTP" flag MUST also be set in the new
PA-OTP-REQUEST.
If the "collect-pin" flag is set, then the token requires a PIN to be
collected by the client. If the "do-not-collect-pin" flag is set in
the otp-tokenInfo from the PA-OTP-CHALLENGE, then the token
represented by the otp-tokenInfo does not require a PIN to be
collected by the client as part of the OTP value. If neither of the
"collect-pin" nor "do-not-collect-pin" flags are set, then PIN
requirements of the token are unspecified. If both flags are set,
then the client SHALL regard the request as invalid.
If the "separate-pin-required" flag is set, then any PIN collected by
the client MUST be included as a UTF-8 string in the otp-pin of the
PA-OTP-REQUEST.
If the token is a connected device, then how the PIN is used to
generate the OTP value will depend on the type of device. However,
if the token is a disconnected device, then it will depend on the
"separate-pin-required" flag. If the flag is not set, then the OTP
value MUST be generated by appending the PIN with the value from the
token entered by the user and, if the flag is set, then the OTP value
MUST be the value from the token.

The clients SHOULD NOT normalize the PIN value or any OTP value
collected from the user or returned by a connected token in any way.
If the "check-digit" flag is set, then any OTP values SHOULD be
decimal and have a Luhn check digit appended [ISOIEC7812]. If the
token is disconnected, then the Client MAY ignore this flag; if the
token is connected, then the Client MUST enforce it. The Client MUST
regard the request as invalid, if otp-format is present and set to
any value other than "decimal".
If an otp-challenge is present in the otp-tokenInfo selected by the
client from the PA-OTP-CHALLENGE, then the OTP value for the token
MUST be generated based on a challenge, if the token is capable of
accepting a challenge. The client MAY ignore the provided challenge
if and only if the token is not capable of including a challenge in
the OTP calculation.
If the "combine" flag is not set in the otp-tokenInfo of the
PA-OTP-CHALLENGE, then the OTP SHALL be calculated based only the
challenge and not the internal state (e.g., time or counter) of the
token. If the "combine" flag is set, then the OTP SHALL be
calculated using both the internal state and the provided challenge,
if that value is obtainable by the client. If the flag is set but
otp-challenge is not present, then the client SHALL regard the
request as invalid.
If token is a connected device, then the use of the challenge will
depend on the type of device but will involve passing the challenge
and the value of the "combine" flag in a token-specific manner to the
token, along with a PIN if collected and the values of otp-length and
otp-format if specified, in order to obtain the OTP value. If the
token is disconnected, then the challenge MUST be displayed to the
user and the value of the "combine" flag MAY be ignored by the
client.
If the OTP value was generated using a challenge that was not sent by
the KDC, then the challenge SHALL be included in the otp-challenge of
the PA-OTP-REQUEST. If the OTP was generated by combining a
challenge (either received from the KDC or generated by the client)
with the token state, then the "combine" flag SHALL be set in the
PA-OTP-REQUEST.

If the "must-encrypt-nonce" flag is set in the otp-tokenInfo, then
the OTP value MUST be used to generate the Client Key and Reply Key
as described in Section 3.6 and MUST NOT be included in the otp-value
field of the PA-OTP-REQUEST. If the flag is not set, then the OTP
value MUST be included in the otp-value field of the PA-OTP-REQUEST
and MUST NOT be used in the key derivation. In this case, the Client
Key and Reply Key SHALL be the same as the Armor Key as described in
Section 3.6; so, if the returning of OTP values in this way does not
conform to local policy on the client (for example, if
KDC-Authentication is required and is not being provided by other
means), then it SHOULD NOT use the token for authentication.
If the supportedHashAlg and iterationCount elements are included in
the otp-tokenInfo, then the client MUST use hashed OTP values in the
generation of the Reply Key and Client Key as described in Section
3.6. The client MUST select the first algorithm from the list that
it supports and the AlgorithmIdentifer [RFC5280] selected MUST be
placed in the hashAlg element of the PA-OTP-REQUEST. However, if
none of the algorithm identifiers conform to local policy
restrictions, then the authentication attempt MUST NOT proceed using
that token. If the value of iterationCount does not conform to local
policy on the client, then the client MAY use a larger value, but
MUST NOT use a lower value. The value of the iteration count used by
the client MUST be returned in the PA-OTP-REQUEST sent to the KDC.
If hashed OTP values are used, then the nonce generated by the client
MUST be as long as the longest key length of the symmetric key types
that it supports and MUST be chosen randomly (see [RFC4086]). The
nonce MUST be included in the PA-OTP-REQUEST, along with the hash
algorithm and iteration count used in the nonce, hashAlg, and
iterationCount fields of the PA-OTP-REQUEST. These fields MUST NOT
be included if hashed OTP values were not used. It is RECOMMENDED
that the iteration count used by the client be chosen in such a way
that it is computationally infeasible/unattractive for an attacker to
brute-force search for the given OTP.
The PA-OTP-REQUEST returned by the client SHOULD include information
on the generated OTP value reported by the OTP token when available
to the client. The otp-time and otp-counter fields of the
PA-OTP-REQUEST SHOULD be used to return the time and counter values
used by the token if available to the client. The otp-format field
MAY be used to report the format of the generated OTP. This field
SHOULD be used if a token can generate OTP values in multiple
formats. The otp-algID field SHOULD be used by the client to report
the algorithm used in the OTP calculation, and the otp-tokenID SHOULD
be used to report the identifier of the OTP token key used if the
information is known to the client.

If the PA-OTP-REQUEST is being sent in response to a PA-OTP-CHALLENGE
that contained an otp-vendor field in the selected otp-tokenInfo,
then the otp-vendor field of the PA-OTP-REQUEST MUST be set to the
same value. If no otp-vendor field was provided by the KDC, then the
field SHOULD be set to the vendor identifier of the token if known to
the client.
The generated Client Key is used by the client to encrypt data to be
included in the encData of the PA-OTP-REQUEST to allow the KDC to
authenticate the user. The key usage for this encryption is
KEY_USAGE_OTP_REQUEST.
o If the PA-OTP-REQUEST is being generated in response to a
PA-OTP-CHALLENGE returned by the KDC, then the client SHALL
encrypt a PA-OTP-ENC-REQUEST containing the value of nonce from
the PA-OTP-CHALLENGE using the same encryption type as the Armor
Key.
o If the PA-OTP-REQUEST is not in response to a PA-OTP-CHALLENGE,
then the client SHALL encrypt a PA-ENC-TS-ENC containing the
current time as in the encrypted timestamp pre-authentication
mechanism [RFC4120].
If the client is working in two-pass mode and so, is not responding
to an initial KDC challenge, then the values of the iteration count
and hash algorithms cannot be obtained from that challenge. The
client SHOULD use any values obtained from a previous
PA-OTP-CHALLENGE or, if no values are available, it MAY use initial
configured values.
3.4. Verifying the Pre-Authentication Data
The KDC validates the pre-authentication data by generating the
Client Key and Reply Key in the same way as the client and using the
generated Client Key to decrypt the value of encData from the
PA-OTP-REQUEST. The generated Reply Key is used to encrypt data in
the AS-REP.
If the otp-value field is included in the PA-OTP-REQUEST, then the
KDC MUST use that value unless the OTP method is required to support
KDC-authentication (see Section 3.2). If the otp-value is not
included in the PA-OTP-REQUEST, then the KDC will need to generate or
obtain the OTP value.
If the otp-pin field is present in the PA-OTP-REQUEST, then the PIN
value has to be value provided by the client. The KDC SHOULD
SASLPrep (Stringprep Profile for User Names and Passwords) [RFC4013]
the value in lookup mode before comparison.

It should be noted that it is anticipated that, as improved string
comparison technologies are standardized, the processing done by the
KDC will change, but efforts will be made to maintain as much
compatibility with SASLprep as possible.
If the otp-challenge field is present, then the OTP was calculated
using that challenge. If the "combine" flag is also set, then the
OTP was calculated using the challenge and the token's current state.
It is RECOMMENDED that the KDC act upon the values of otp-time,
otp-counter, otp-format, otp-algID, and otp-tokenID if they are
present in the PA-OTP-REQUEST. If the KDC receives a request
containing these values, but cannot act upon them, then they MAY be
ignored.
The KDC generates the Client Key and Reply Key as described in
Section 3.6 from the OTP value using the nonce, hash algorithm, and
iteration count if present in the PA-OTP-REQUEST. The KDC MUST fail
the request with KDC_ERR_INVALID_HASH_ALG, if the KDC requires hashed
OTP values and the hashAlg field was not present in the
PA-OTP-REQUEST or if the value of this field does not conform to
local KDC policy. Similarly, the KDC MUST fail the request with
KDC_ERR_INVALID_ITERATION_COUNT, if the value of the iterationCount
included in the PA-OTP-REQUEST does not conform to local KDC policy
or is less than that specified in the PA-OTP-CHALLENGE. In addition,
the KDC MUST fail the authentication request with
KDC_ERR_PIN_REQUIRED, if it requires a separate PIN to the OTP value
and an otp-pin was not included in the PA-OTP-REQUEST. The above
error codes are defined as follows:
KDC_ERR_INVALID_HASH_ALG 94
KDC_ERR_INVALID_ITERATION_COUNT 95
KDC_ERR_PIN_REQUIRED 97
The generated Client Key is then used to decrypt the encData from the
PA-OTP-REQUEST. If the client response was sent as a result of a
PA-OTP-CHALLENGE, then the decrypted data will be a
PA-OTP-ENC-REQUEST and the client authentication MUST fail with
KDC_ERR_PREAUTH_FAILED if the nonce value from the PA-OTP-ENC-REQUEST
is not the same as the nonce value sent in the PA-OTP-CHALLENGE. If
the response was not sent as a result of a PA-OTP-CHALLENGE, then the
decrypted value will be a PA-ENC-TS-ENC, and the authentication
process will be the same as with classic encrypted timestamp
pre-authentication [RFC4120].
The KDC MUST fail the request with KDC_ERR_ETYPE_NOSUPP, if the
encryption type used by the client in the encData does not conform to
KDC policy.

If authentication fails due to the hash algorithm, iteration count,
or encryption type used by the client, then the KDC SHOULD return a
PA-OTP-CHALLENGE with the required values in the error response. If
the authentication fails due to the token state on the server is no
longer being synchronized with the token used, then the KDC MAY
return a PA-OTP-CHALLENGE with the "nextOTP" flag set as described in
Section 2.4.
If, during the authentication process, the KDC determines that the
user's PIN has been changed, then it SHOULD include a
PA-OTP-PIN-CHANGE in the response, as described in Section 2.3,
containing the new PIN value. The KDC MAY also include the new PIN's
expiration time and the expiration time of the OTP account within the
last-req field of the PA-OTP-PIN-CHANGE. (These fields can be used
by the KDC to handle cases where the account related to the user's
OTP token has a different expiration time to the user's Kerberos
account.) If the KDC determines that the user's PIN or OTP account
are about to expire, it MAY return a PA-OTP-PIN-CHANGE with that
information. Finally, if the KDC determines that the user's PIN has
expired, then it SHOULD return a KRB-ERROR of type
KDC_ERR_PIN_EXPIRED as described in Section 2.33.5. Confirming the Reply Key Change
If the pre-authentication data was successfully verified, then, in
order to support mutual authentication, the KDC SHALL respond to the
client's PA-OTP-REQUEST by using the generated Reply Key to encrypt
the data in the AS-REP. The client then uses its generated Reply Key
to decrypt the encrypted data and MUST NOT continue with the
authentication process, if decryption is not successful.
3.6. Reply Key Generation
In order to authenticate the user, the client and KDC need to
generate two encryption keys:
o The Client Key to be used by the client to encrypt and by the KDC
to decrypt the encData in the PA-OTP-REQUEST.
o The Reply Key to be used in the standard manner by the KDC to
encrypt data in the AS-REP.
The method used to generate the two keys will depend on the OTP
algorithm.
o If the OTP value is included in the otp-value of the PA-OTP-
REQUEST, then the two keys SHALL be the same as the Armor Key
(defined in [RFC6113]).

o If the OTP value is not included in the otp-value of the
PA-OTP-REQUEST, then the two keys SHALL be derived from the Armor
Key and the OTP value as described below.
If the OTP value is not included in the PA-OTP-REQUEST, then the
Reply Key and Client Key SHALL be generated using the KRB-FX-CF2
algorithm from [RFC6113] as follows:
Client Key = KRB-FX-CF2(K1, K2, O1, O2)
Reply Key = KRB-FX-CF2(K1, K2, O3, O4)
The octet string parameters, O1, O2, O3, and O4 shall be the ASCII
string "OTPComb1", "OTPComb2", "OTPComb3", and "OTPComb4" as shown
below:
{0x4f, 0x54, 0x50, 0x43, 0x6f, 0x6d, 0x62, 0x31}
{0x4f, 0x54, 0x50, 0x43, 0x6f, 0x6d, 0x62, 0x32}
{0x4f, 0x54, 0x50, 0x43, 0x6f, 0x6d, 0x62, 0x33}
{0x4f, 0x54, 0x50, 0x43, 0x6f, 0x6d, 0x62, 0x34}
The first input key, K1, SHALL be the Armor Key and so, as described
in Section 5.1 of [RFC6113], the enctypes of the generated Client Key
and Reply Key will be the same as the enctype of Armor Key. The
second input key, K2, shall be derived from the OTP value using
string-to-key (defined in [RFC3961]) as described below.
If the hash of the OTP value is to be used, then K2 SHALL be derived
as follows:
o An initial hash value, H, is generated:
H = hash(realm|nonce|OTP)
Where:
* "|" denotes concatenation.
* hash is the hash algorithm selected by the client.
* realm is the name of the server's realm as carried in the realm
field of the AS-REQ (not including the tag and length from the
DER encoding).
* nonce is the value of the random nonce value generated by the
client and carried in the nonce field of the PA-OTP-REQUEST
(not including the tag and length from the DER encoding).
* If the OTP format is decimal, hexadecimal, or alphanumeric,
then OTP is the value of the OTP generated as described in
Section 3.3 with SASLprep [RFC4013] applied in lookup mode;
otherwise, it is the unnormalized OTP value.

o The initial hash value is then hashed iterationCount-1 times to
produce a final hash value, H' (where iterationCount is the value
from the PA-OTP-REQUEST).
H' = hash(hash(...(iterationCount-1 times)...(H)))
o The value of K2 is then derived from the Base64 [RFC2045] encoding
of this final hash value.
K2 = string-to-key(Base64(H')|"Krb-preAuth")
If the hash value is not used, then K2 SHALL be derived from the
base64 encoding of the OTP value.
K2 = string-to-key(Base64(OTP)|"Krb-preAuth")
The enctype used for string-to-key SHALL be that of the Armor Key and
the salt and any additional parameters for string-to-key MAY be
provided by the KDC in the PA-OTP-CHALLENGE. If the salt and
string-to-key parameters are not provided, then the default values
defined for the particular enctype SHALL be used.
If the strengthen-key is present in KrbFastResponse, then it is
combined with the Reply Key to generate the final AS-REQ as described
in [RFC6113]. The strengthen-key does not influence the Client Key.
4. OTP Kerberos Message Types
4.1. PA-OTP-CHALLENGE
The padata-type PA-OTP-CHALLENGE is returned by the KDC to the client
in the enc-fast-rep of a PA-FX-FAST-REPLY in the PA-DATA of a
KRB-ERROR when OTP pre-authentication is required. The corresponding
padata-value field contains the Distinguished Encoding Rules (DER)
[X.680] and [X.690] encoding of a PA-OTP-CHALLENGE containing a
server-generated nonce and information for the client on how to
generate the OTP.
PA-OTP-CHALLENGE 141
PA-OTP-CHALLENGE ::= SEQUENCE {
nonce [0] OCTET STRING,
otp-service [1] UTF8String OPTIONAL,
otp-tokenInfo [2] SEQUENCE (SIZE(1..MAX)) OF
OTP-TOKENINFO,
salt [3] KerberosString OPTIONAL,
s2kparams [4] OCTET STRING OPTIONAL,
...

otp-tokenInfo
This element MUST be included, and it is a sequence of one or more
OTP-TOKENINFO objects containing information on the token or
tokens that the user can use for the authentication and how the
OTP value is to be generated using those tokens. If a single
OTP-TOKENINFO object is included, then only a single token is
acceptable by the KDC and any OTP value generated by the client
MUST be generated according to the information contained within
that element. If more than one OTP-TOKENINFO object is included,
then the OTP value MUST be generated according to the information
contained within one of those objects.
flags
If the "nextOTP" flag is set, then the OTP SHALL be based on
the next token "state" rather than the one used in the previous
authentication. As an example, for a time-based token, this
means the next time slot and for an event-based token, this
could mean the next counter value. If the "nextOTP" flag is
set, then there MUST only be a single otp-tokenInfo element in
the PA-OTP-CHALLENGE.
The "combine" flag controls how the challenge included in
otp-challenge shall be used. If the flag is set, then OTP
SHALL be calculated using the challenge from otp-challenge and
the internal token state (e.g., time or counter). If the
"combine" flag is not set, then the OTP SHALL be calculated
based only on the challenge. If the flag is set and otp-
challenge is not present, then the request SHALL be regarded as
invalid.
If the "do-not-collect-pin" flag is set, then the token
represented by the current otp-tokenInfo does not require a PIN
to be collected as part of the OTP. If the "collect-pin" flag
is set, then the token requires a PIN. If neither flag is set,
then whether or not a PIN is required is unspecified. The
flags are mutually exclusive and so both flags MUST NOT be set,
or the client MUST regard the request as invalid.
If the "must-encrypt-nonce" flag is set, then the OTP value
MUST NOT be included in the otp-value field of the
PA-OTP-REQUEST, but instead the OTP value MUST be used in the
generation of the Reply Key and Client Key as described in
Section 3.6.
If the "separate-pin-required" flag is set, then the PIN
collected by the client SHOULD NOT be used in the generation of
the OTP value and SHOULD be returned in the otp-pin field of
the PA-OTP-REQUEST.

The "check-digit" flag controls whether or not the OTP values
generated by the token need to include a Luhn check digit
[ISOIEC7812]. If the token is disconnected, then the Client
MAY ignore this flag; if this flag is set and the token is
connected, then the OTP MUST be a decimal with a check digit
appended.
otp-vendor
Use of this field is OPTIONAL, but MAY be used by the KDC to
identify the vendor of the OTP token to be used.
otp-challenge
The otp-challenge is used by the KDC to send a challenge value
for use in the OTP calculation. The challenge is an OPTIONAL
octet string that SHOULD be uniquely generated for each request
in which it is present. When the challenge is not present, the
OTP will be calculated on the current token state only. The
client MAY ignore a provided challenge if and only if the OTP
token the client is interacting with is not capable of
including a challenge in the OTP calculation. In this case,
KDC policies will determine whether or not to accept a provided
OTP value.
otp-length
Use of this field is OPTIONAL, but MAY be used by the KDC to
specify the desired length of the generated OTP. For example,
this field could be used when a token is capable of producing
OTP values of different lengths. If the format of the OTP is
'decimal', 'hexidecimal', or 'alphanumeric', then this value
indicates the desired length in digits/characters; if the OTP
format is 'binary', then this value indicates the desired
length in octets; and if the OTP format is 'base64', then this
value indicates the desired length of the unencoded OTP value
in octets.
otp-format
Use of this field is OPTIONAL, but MAY be used by the KDC to
specify the desired format of the generated OTP value. For
example, this field could be used when a token is capable of
producing OTP values of different formats.
otp-tokenID
Use of this field is OPTIONAL, but MAY be used by the KDC to
identify which token key should be used for the authentication.
For example, this field could be used when a user has been
issued multiple token keys by the same server.

otp-algID
Use of this field is OPTIONAL, but MAY be used by the KDC to
identify the algorithm to use when generating the OTP. The
value of this field MUST be a URI [RFC3986] and SHOULD be
obtained from the Portable Symmetric Key Container (PSKC)
algorithm registry [RFC6030].
supportedHashAlg
If present, then a hash of the OTP value MUST be used in the
key derivation rather than the plain text value. Each
AlgorithmIdentifier identifies a hash algorithm that is
supported by the KDC in decreasing order of preference. The
client MUST select the first algorithm from the list that it
supports. Support for SHA-256 by both the client and KDC is
REQUIRED. The AlgorithmIdentifier selected by the client MUST
be placed in the hashAlg element of the PA-OTP-REQUEST.
iterationCount
The minimum value of the iteration count to be used by the
client when hashing the OTP value. This value MUST be present
if supportedHashAlg is present and otherwise MUST NOT be
present. If the value of this element does not conform to
local policy on the client, then the client MAY use a larger
value but MUST NOT use a lower value. The value of the
iteration count used by the client MUST be returned in the
PA-OTP-REQUEST sent to the KDC.
salt
The salt value to be used in string-to-key when used to calculate
the keys as described in Section 3.6.
s2kparams
Any additional parameters required by string-to-key as described
in Section 3.6.
4.2. PA-OTP-REQUEST
The padata-type PA-OTP-REQUEST is sent by the client to the KDC in
the KrbFastReq padata of a PA-FX-FAST-REQUEST that is included in the
PA-DATA of an AS-REQ. The corresponding padata-value field contains
the DER encoding of a PA-OTP-REQUEST.
The message contains pre-authentication data encrypted by the client
using the generated Client Key and optional information on how the
OTP was generated. It may also, optionally, contain the generated
OTP value.

PA-OTP-REQUEST 142
PA-OTP-REQUEST ::= SEQUENCE {
flags [0] OTPFlags,
nonce [1] OCTET STRING OPTIONAL,
encData [2] EncryptedData,
-- PA-OTP-ENC-REQUEST or PA-ENC-TS-ENC
-- Key usage of KEY_USAGE_OTP_REQUEST
hashAlg [3] AlgorithmIdentifier OPTIONAL,
iterationCount [4] Int32 OPTIONAL,
otp-value [5] OCTET STRING OPTIONAL,
otp-pin [6] UTF8String OPTIONAL,
otp-challenge [7] OCTET STRING (SIZE(1..MAX)) OPTIONAL,
otp-time [8] KerberosTime OPTIONAL,
otp-counter [9] OCTET STRING OPTIONAL,
otp-format [10] OTPFormat OPTIONAL,
otp-tokenID [11] OCTET STRING OPTIONAL,
otp-algID [12] AnyURI OPTIONAL,
otp-vendor [13] UTF8String OPTIONAL,
...
}
KEY_USAGE_OTP_REQUEST 45
PA-OTP-ENC-REQUEST ::= SEQUENCE {
nonce [0] OCTET STRING,
...
}
flags
This field MUST be present.
If the "nextOTP" flag is set, then the OTP was calculated based on
the next token "state" rather than the current one. This flag
MUST be set if and only if it was set in a corresponding
PA-OTP-CHALLENGE.
If the "combine" flag is set, then the OTP was calculated based on
a challenge and the token state.
No other OTPFlag bits are applicable and MUST be ignored by the
KDC.
nonce
This field MUST be present if a hashed OTP value was used as input
to string-to-key (see Section 3.6) and MUST NOT be present
otherwise. If present, it MUST contain the nonce value generated
by the client and used in the generation of hashed OTP values as

described in Section 3.6. This nonce string MUST be as long as
the longest key length of the symmetric key types that the client
supports and MUST be chosen randomly.
encData
This field MUST be present and MUST contain the pre-authentication
data encrypted under the Client Key with a key usage of
KEY_USAGE_OTP_REQUEST. If the PA-OTP-REQUEST is sent as a result
of a PA-OTP-CHALLENGE, then this MUST contain a PA-OTP-ENC-REQUEST
with the nonce from the PA-OTP-CHALLENGE. If no challenge was
received, then this MUST contain a PA-ENC-TS-ENC.
hashAlg
This field MUST be present if a hashed OTP value was used as input
to string-to-key (see Section 3.6) and MUST NOT be present
otherwise. If present, it MUST contain the AlgorithmIdentifier of
the hash algorithm used. If the PA-OTP-REQUEST is sent as a
result of a PA-OTP-CHALLENGE, then the AlgorithmIdentifer MUST be
the first one supported by the client from the supportedHashAlg of
the PA-OTP-CHALLENGE.
iterationCount
This field MUST be present if a hashed OTP value was used as input
to string-to-key (see Section 3.6) and MUST NOT be present
otherwise. If present, it MUST contain the iteration count used
when hashing the OTP value. If the PA-OTP-REQUEST is sent as a
result of a PA-OTP-CHALLENGE, then the value MUST NOT be less that
specified in the PA-OTP-CHALLENGE.
otp-value
The generated OTP value. This value MUST NOT be present if the
"must-encrypt-nonce" flag was set in the PA-OTP-CHALLENGE.
otp-pin
The OTP PIN value entered by the user. This value MUST NOT be
present unless the "separate-pin-required" flag was set in the
PA-OTP-CHALLENGE.
otp-challenge
Value used by the client in the OTP calculation. It MUST be sent
to the KDC if and only if the value would otherwise be unknown to
the KDC (for example, the token- or client-modified or generated
challenge).

otp-time
This field MAY be included by the client to carry the time value
as reported by the OTP token. Use of this element is OPTIONAL,
but it MAY be used by a client to simplify the OTP calculations
carried out by the KDC. It is RECOMMENDED that the KDC act upon
this value if it is present in the request and it is capable of
using it in the generation of the OTP value.
otp-counter
This field MAY be included by the client to carry the token
counter value, as reported by the OTP token. Use of this element
is OPTIONAL, but it MAY be used by a client to simplify the OTP
calculations carried out by the KDC. It is RECOMMENDED that the
KDC act upon this value if it is present in the request and it is
capable of using it in the generation of the OTP value.
otp-format
This field MAY be used by the client to send the format of the
generated OTP as reported by the OTP token. Use of this element
is OPTIONAL, but it MAY be used by the client to simplify the OTP
calculations carried out by the KDC. It is RECOMMENDED that the
KDC act upon this value, if it is present in the request and it is
capable of using it in the generation of the OTP value.
otp-tokenID
This field MAY be used by the client to send the identifier of the
token key used, as reported by the OTP token. Use of this field
is OPTIONAL, but MAY be used by the client to simplify the
authentication process by identifying a particular token key
associated with the user. It is RECOMMENDED that the KDC act upon
this value, if it is present in the request and it is capable of
using it in the generation of the OTP value.
otp-algID
This field MAY be used by the client to send the identifier of the
OTP algorithm used, as reported by the OTP token. Use of this
element is OPTIONAL, but it MAY be used by the client to simplify
the OTP calculations carried out by the KDC. It is RECOMMENDED
that the KDC act upon this value, if it is present in the request
and it is capable of using it in the generation of the OTP value.
The value of this field MUST be a URI [RFC3986] and SHOULD be
obtained from the PSKC algorithm registry [RFC6030].

otp-vendor
If the PA-OTP-REQUEST is being sent in response to a
PA-OTP-CHALLENGE that contained an otp-vendor field in the
selected otp-tokenInfo, then this field MUST be set to the same
value; otherwise, this field SHOULD be set to the vendor
identifier of the token, if known to the client. It is
RECOMMENDED that the KDC act upon this value if it is present in
the request and it is capable of using it in the generation of the
OTP value.
4.3. PA-OTP-PIN-CHANGE
The padata-type PA-OTP-PIN-CHANGE is returned by the KDC in the
enc-fast-rep of a PA-FX-FAST-REPLY in the AS-REP if the user must
change their PIN, if the user's PIN has been changed, or to notify
the user of the PIN's expiry time.
The corresponding padata-value field contains the DER encoding of a
PA-OTP-PIN-CHANGE.
PA-OTP-PIN-CHANGE 144
PA-OTP-PIN-CHANGE ::= SEQUENCE {
flags [0] PinFlags,
pin [1] UTF8String OPTIONAL,
minLength [2] INTEGER OPTIONAL,
maxLength [3] INTEGER OPTIONAL,
last-req [4] LastReq OPTIONAL,
format [5] OTPFormat OPTIONAL,
...
}
PinFlags ::= KerberosFlags
-- reserved(0),
-- systemSetPin(1),
-- mandatory(2)
flags
The "systemSetPin" flag is used to indicate the type of PIN change
that is taking place. If the flag is set, then the user's PIN has
been changed for the user by the system. If the flag is not set,
then the user's PIN needs to be changed by the user.
If the "systemSetPin" flag is not set and the "mandatory" flag is
set, then user PIN change is required before the next
authentication using the current OTP token. If the "mandatory"
flag is not set, then the user PIN change is optional. If the

"systemSetPin" flag is set, then the "mandatory" flag has no
meaning and SHOULD be ignored by the client.
pin
The pin field is used to carry a new PIN value. If the
"systemSetPin" flag is set, then the pin field is used to carry
the new PIN value set for the user and MUST be present. If the
"systemSetPin" flag is not set, then use of this field is OPTIONAL
and MAY be used to carry a system-generated PIN that MAY be used
by the user when changing the PIN.
minLength and maxLength
Use of the minLength and maxLength fields is OPTIONAL. If the
"systemSetPin" flag is not set, then these fields MAY be included
to pass restrictions on the size of the user-selected PIN.
last-req
Use of the last-req field (as defined in Section 5.4.2 of
[RFC4120])) is OPTIONAL, but MAY be included with an lr-type of 6
to carry PIN expiration information.
* If the "systemSetPin" flag is set, then the expiration time
MUST be that of the new system-set PIN.
* If the "systemSetPin" flag is not set, then the expiration time
MUST be that of the current PIN of the token used in the
authentication.
The element MAY also be included with an lr-type of 7 to indicate
when the OTP account will expire.
format
The format element MAY be included by the KDC to carry format
restrictions on the new PIN.
* If the "systemSetPin" flag is set, then the element MUST
describe the format of the new system-generated PIN.
* If the "systemSetPin" flag is not set, then the element MUST
describe restrictions on any new user-generated PIN.
5. IANA Considerations
The OTP algorithm identifier URIs used as otp-algID values in the
PA-OTP-CHALLENGE described in Section 4.1 and the PA-OTP-REQUEST
described in Section 4.2 have been registered in the "Algorithm URI
Registry and Related PSKC Profiles" registry [RFC6030].

The following pre-authentication types are defined in this document:
PA-OTP-CHALLENGE 141
PA-OTP-REQUEST 142
PA-OTP-PIN-CHANGE 144
Note that PA-OTP-CONFIRM (143) has been marked as OBSOLETE per this
document.
These values are currently registered in a registry created by
[RFC6113], but the entries have been updated to refer to this
document.
The following error codes and key usage values are defined in this
document:
KDC_ERR_INVALID_HASH_ALG 94
KDC_ERR_INVALID_ITERATION_COUNT 95
KDC_ERR_PIN_EXPIRED 96
KDC_ERR_PIN_REQUIRED 97
KEY_USAGE_OTP_REQUEST 45
These values are currently not managed by IANA and have not been
accounted for. There is currently work in progress [LHA10] to define
IANA registries and a registration process for these values.
6. Security Considerations
6.1. Man-in-the-Middle Attacks
In the system described in this document, the OTP pre-authentication
protocol is tunneled within the FAST Armor channel provided by the
pre-authentication framework. As described in [ASNINY02], tunneled
protocols are potentially vulnerable to man-in-the-middle (MITM)
attacks if the outer tunnel is compromised, and it is generally
considered good practice in such cases to bind the inner encryption
to the outer tunnel.
In order to mitigate against such attacks, the proposed system uses
the outer Armor Key in the derivation of the inner Client and Reply
keys and so achieves crypto-binding to the outer channel.
As described in Section 5.4 of [RFC6113], FAST can use an anonymous
Ticket-Granting Ticket (TGT) obtained using anonymous Public Key
Cryptography for Initial Authentication in Kerberos (PKINIT)
[RFC6112] [RFC4556] as the Armor Key. However, the current anonymous
PKINIT proposal is open to MITM attacks since the attacker

can choose a session key such that the session key between the MITM
and the real KDC is the same as the session key between the client
and the MITM.
As described in Section 3.6, if the OTP value is not being sent to
the KDC, then the Armor Key is used along with the OTP value in the
generation of the Client Key and Reply Key. If the Armor Key is
known, then the only entropy remaining in the key generation is
provided by the OTP value. If the OTP algorithm requires that the
OTP value be sent to the KDC, then it is sent encrypted within the
tunnel provided by the FAST Armor and so, is exposed to the attacker
if the attacker has the Armor Key.
Therefore, unless the identity of the KDC has been verified,
anonymous PKINIT SHALL NOT be used with OTP algorithms that require
the OTP value to be sent to the KDC. In addition, the security
considerations should be carefully considered before anonymous PKINIT
is used with other algorithms such as those with short OTP values.
Careful consideration should also be made if host key armor is used
to provide the KDC-authentication facility with OTP algorithms where
the OTP value is sent within the otp-value field of the
PA-OTP-REQUEST since compromised host keys would allow an attacker to
impersonate the KDC.
6.2. Reflection
The four-pass system described above is a challenge-response
protocol, and such protocols are potentially vulnerable to reflection
attacks. No such attacks are known at this point, but to help
mitigate against such attacks, the system uses different keys to
encrypt the client and server nonces.
6.3. Denial-of-Service Attacks
The protocol supports the use of an iteration count in the generation
of the Client and Reply keys, and the client can send the number of
iterations used as part of the PA-OTP-REQUEST. This could open the
KDC up to a denial-of-service attack if a large value for the
iteration count was specified by the attacker. It is therefore,
particularly important that, as described in Section 3.4, the KDC
reject any authentication requests where the iteration count is above
a maximum value specified by local policy.

6.4. Replay
In the four-pass version of this protocol, the client encrypts a
KDC-generated nonce, so replay can be detected by the KDC. The
two-pass version of the protocol does not involve a server nonce; the
client instead encrypts a timestamp, and therefore is not protected
from replay in this way, but it will instead require some other
mechanism, such as an OTP-server-based system or a timestamp-based
replay cache on the KDC.
As described in Section 5.2 of [RFC6113], a client cannot be certain
that it will use the same KDC for all messages in a conversation.
Therefore, the client cannot assume that the PA-OTP-REQUEST will be
sent to the same KDC that issued the PA-OTP-CHALLENGE. In order to
support this, a KDC implementing this protocol requires a means of
sharing session state. However, doing this does introduce the
possibility of a replay attack where the same response is sent to
multiple KDCs.
In the case of time- or event-based tokens or server-generated
challenges, protection against replay may be provided by the OTP
server being used if that server is capable of keeping track of the
last used value. This protection therefore relies upon the
assumption that the OTP server being used in this protocol is either
not redundant or involves a commit protocol to synchronize between
replicas. If this does not hold for an OTP server being used, then
the system may be vulnerable to replay attacks.
However, OTP servers may not be able to detect replay of OTPs
generated using only a client-generated challenge; since, the KDC
would not be able to detect replay in two-pass mode, it is
recommended that the use of OTPs generated from only a
client-generated challenge (that is, not in combination with some
other factor such as time) should not be supported in two-pass mode.
6.5. Brute-Force Attack
A compromised or hostile KDC may be able to obtain the OTP value used
by the client via a brute-force attack. If the OTP value is short,
then the KDC could iterate over the possible OTP values until a
Client Key is generated that can decrypt the encData sent in the
PA-OTP-REQUEST.

As described in Section 3.6, an iteration count can be used in the
generation of the Client Key and the value of the iteration count can
be controlled by local client policy. Use of this iteration count
can make it computationally infeasible/unattractive for an attacker
to brute-force search for the given OTP within the lifetime of that
OTP.
If PINs contain international characters, similar looking or similar
functioning characters may be mapped together. For example, the
combined and decomposed forms of accented characters will typically
be treated the same. Users who attempt to exploit artifacts of
international characters to improve the strength of their PINs may
experience false positives in the sense that PINs they intended to be
distinct are not actually distinct. This decision was made in order
to improve usability across the widest variety of input methods.
Users can choose other methods to add strength to PINs.
6.6. FAST Facilities
The secret used to generate the OTP is known only to the client and
the KDC, so successful decryption of the encrypted nonce by the KDC
authenticates the user. If the OTP value is used in the Reply Key
generation, then successful decryption of the encrypted nonce by the
client proves that the expected KDC replied. The Reply Key is
replaced by either a key generated from the OTP and Armor Key or by
the Armor Key. This FAST factor therefore, provides the following
facilities: client-authentication, replacing-reply-key, and,
depending on the OTP algorithm, KDC-authentication.
7. Acknowledgments
Many significant contributions were made to this document by RSA
employees, but special thanks go to Magnus Nystrom, John Linn,
Richard Zhang, Piers Bowness, Robert Philpott, Robert Polansky, and
Boris Khoutorski.
Many valuable suggestions were also made by members of the Kerberos
Working Group, but special thanks go to Larry Zhu, Jeffrey Hutzelman,
Tim Alsop, Henry Hotz, Nicolas Williams, Sam Hartman, Frank Cusak,
Simon Josefsson, Greg Hudson, and Linus Nordberg.
I would also like to thank Tim Alsop and Srinivas Cheruku of
CyberSafe for many valuable review comments.

Appendix B. Examples of OTP Pre-Authentication Exchanges
This section is non-normative.
B.1. Four-Pass Authentication
In this mode, the client sends an initial AS-REQ to the KDC that does
not contain a PA-OTP-REQUEST and the KDC responds with a KRB-ERROR
containing a PA-OTP-CHALLENGE.
In this example, the user has been issued with a connected,
time-based token, and the KDC requires hashed OTP values in the key
generation with SHA-384 as the preferred hash algorithm and a minimum
of 1024 iterations. The local policy on the client supports SHA-256
and requires 100,000 iterations of the hash of the OTP value.
The basic sequence of steps involved is as follows:
1. The client obtains the user name of the user.
2. The client sends an initial AS-REQ to the KDC that does not
contain a PA-OTP-REQUEST.
3. The KDC determines that the user identified by the AS-REQ
requires OTP authentication.
4. The KDC constructs a PA-OTP-CHALLENGE as follows:
nonce
A randomly generated value.
otp-service
A string that can be used by the client to assist the user in
locating the correct token.
otp-tokenInfo
Information about how the OTP should be generated from the
token.
flags
must-encrypt-nonce and collect-pin bits set
supportedHashAlg
AlgorithmIdentifiers for SHA-384, SHA-256, and SHA-1
iterationCount
1024

5. The KDC returns a KRB-ERROR with an error code of
KDC_ERR_PREAUTH_REQUIRED and the PA-OTP-CHALLENGE in the e-data.
6. The client displays the value of otp-service and prompts the
user to connect the token.
7. The client collects a PIN from the user.
8. The client obtains the current OTP value from the token using
the PIN and records the time as reported by the token.
9. The client generates the Client Key and Reply Key as described
in Section 3.6 using SHA-256 from the list of algorithms sent by
the KDC, the iteration count of 100,000 as required by local
policy, and a randomly generated nonce.
10. The client constructs a PA-OTP-REQUEST as follows:
flags
0
nonce
The randomly generated value.
encData
An EncryptedData containing a PA-OTP-ENC-REQUEST encrypted
under the Client Key with a key usage of
KEY_USAGE_OTP_REQUEST and the encryption type of the Armor
Key. The PA-OTP-ENC-REQUEST contains the nonce from the
PA-OTP-CHALLENGE.
hashAlg
SHA-256
iterationCount
100,000
otp-time
The time used in the OTP calculation as reported by the OTP
token.
11. The client encrypts the PA-OTP-REQUEST within the enc-fast-req
of a PA-FX-FAST-REQUEST.
12. The client sends an AS-REQ to the KDC containing the
PA-FX-FAST-REQUEST within the padata.

13. The KDC validates the padata in the PA-OTP-REQUEST by performing
the following steps:
* Generates the Client Key and Reply Key from the OTP value for
the user identified in the AS-REQ, using an iteration count
of 100,000, a hash algorithm of SHA-256, and the nonce as
specified in the PA-OTP-REQUEST.
* Uses the generated Client Key to decrypt the
PA-OTP-ENC-REQUEST in the encData of the PA-OTP-REQUEST.
* Authenticates the user by comparing the nonce value from the
decrypted PA-OTP-ENC-REQUEST to that sent in the
corresponding PA-OTP-CHALLENGE.
14. The KDC constructs a TGT for the user.
15. The KDC returns an AS-REP to the client, encrypted using the
Reply Key, containing the TGT and padata with the
PA-FX-FAST-REPLY.
16.
The client authenticates the KDC and verifies the Reply Key
change. The client uses the generated Reply Key to decrypt the
encrypted data in the AS-REP.
B.2. Two-Pass Authentication
In this mode, the client includes a PA-OTP-REQUEST within a
PA-FX-FAST-REQUEST padata of the initial AS-REQ sent to the KDC.
In this example, the user has been issued a hand-held token, so, none
of the OTP generation parameters (otp-length, etc.) are included in
the PA-OTP-REQUEST. The KDC does not require hashed OTP values in
the key generation.
It is assumed that the client has been configured with the following
information or has obtained it from a previous PA-OTP-CHALLENGE.
o The OTP value must not be carried in the otp-value.
o The hashed OTP values are not required.
The basic sequence of steps involved is as follows:
1. The client obtains the user name and OTP value from the user.

2. The client generates the Client Key and Reply Key using unhashed
OTP values as described in Section 3.6.
3. The client constructs a PA-OTP-REQUEST as follows:
flags
0
encData
An EncryptedData containing a PA-ENC-TS-ENC encrypted under
the Client Key with a key usage of KEY_USAGE_OTP_REQUEST and
an encryption type of the Armor Key. The PA-ENC-TS-ENC
contains the current client time.
4. The client encrypts the PA-OTP-REQUEST within the enc-fast-req
of a PA-FX-FAST-REQUEST.
5. The client sends an AS-REQ to the KDC containing the
PA-FX-FAST-REQUEST within the padata.
6. The KDC validates the padata by performing the following steps:
* Generates the Client Key and Reply Key from the unhashed OTP
value for the user identified in the AS-REQ.
* Uses the generated Client Key to decrypt the PA-ENC-TS-ENC in
the encData of the PA-OTP-REQUEST.
* Authenticates the user using the timestamp in the standard
manner.
7. The KDC constructs a TGT for the user.
8. The KDC returns an AS-REP to the client, encrypted using the
Reply Key, containing the TGT and padata with the
PA-FX-FAST-REPLY.
9. The client authenticates the KDC and verifies the key change.
The client uses the generated Reply Key to decrypt the encrypted
data in the AS-REP.

B.3. PIN Change
This exchange follows from the point where the KDC receives the
PA-OTP-REQUEST from the client in the examples in Appendix B.1 and
Appendix B.2. During the validation of the pre-authentication data
(whether encrypted nonce or encrypted timestamp), the KDC determines
that the user's PIN has expired and so, must be changed. The KDC
therefore, includes a PA-OTP-PIN-CHANGE in the AS-REP.
In this example, the KDC does not generate PIN values for the user
but requires that the user generate a new PIN that is between 4 and 8
characters in length. The actual PIN change is handled by a PIN
change service.
The basic sequence of steps involved is as follows:
1. The client constructs and sends a PA-OTP-REQUEST to the KDC as
described in the previous examples.
2. The KDC validates the pre-authentication data and authenticates
the user as in the previous examples but determines that the
user's PIN has expired.
3. The KDC constructs a PA-OTP-PIN-CHANGE as follows:
flags
0
minLength
4
maxLength
8
4. The KDC encrypts the PA-OTP-PIN-CHANGE within the enc-fast-rep
of a PA-FX-FAST-REPLY.
5. The KDC returns a KRB-ERROR to the client of type
KDC_ERR_PIN_EXPIRED with padata containing the PA-FX-FAST-REPLY.
6. The client authenticates to the PIN change service and changes
the user's PIN.
7. The client sends a second AS-REQ to the KDC containing a PA-OTP-
REQUEST constructed using the new PIN.
8. The KDC responds with an AS-REP containing a TGT.

B.4. Resynchronization
This exchange follows from the point where the KDC receives the
PA-OTP-REQUEST from the client. During the validation of the
pre-authentication data (whether encrypted nonce or encrypted
timestamp), the KDC determines that the local record of the token's
state needs to be resynchronized with the token. The KDC therefore,
includes a KRB-ERROR containing a PA-OTP-CHALLENGE with the "nextOTP"
flag set.
The sequence of steps below follows is a variation of the four pass
examples shown in Appendix B.1 but the same process would also work
in the two-pass case.
1. The client constructs and sends a PA-OTP-REQUEST to the KDC as
described in the previous examples.
2. The KDC validates the pre-authentication data and authenticates
the user as in the previous examples, but determines that user's
token requires resynchronizing.
3. KDC constructs a PA-OTP-CHALLENGE as follows:
nonce
A randomly generated value.
otp-service
Set to a string that can be used by the client to assist the
user in locating the correct token.
otp-tokenInfo
Information about how the OTP should be generated from the
token.
flags
must-encrypt-nonce, collect-pin, and nextOTP bits set
supportedHashAlg
AlgorithmIdentifiers for SHA-256 and SHA-1
iterationCount
1024
4. KDC returns a KRB-ERROR with an error code of
KDC_ERR_PREAUTH_REQUIRED and the PA-OTP-CHALLENGE in the e-data.
5. The client obtains the next OTP value from the token and records
the time as reported by the token.

6. The client generates the Client Key and Reply Key as described
in Section 3.6 using SHA-256 from the list of algorithms sent by
the KDC, the iteration count of 100,000 as required by local
policy, and a randomly generated nonce.
7. The client constructs a PA-OTP-REQUEST as follows:
flags
nextOTP bit set
nonce
The randomly generated value.
encData
An EncryptedData containing a PA-OTP-ENC-REQUEST encrypted
under the Client Key with a key usage of
KEY_USAGE_OTP_REQUEST and the encryption type of the Armor
Key. The PA-OTP-ENC-REQUEST contains the nonce from the
PA-OTP-CHALLENGE.
hashAlg
SHA-256
iterationCount
100,000
otp-time
The time used in the OTP calculation as reported by the OTP
token.
8. The client encrypts the PA-OTP-REQUEST within the enc-fast-req
of a PA-FX-FAST-REQUEST.
9. The client sends an AS-REQ to the KDC containing the
PA-FX-FAST-REQUEST within the padata.
10. The authentication process now proceeds as with the classic
sequence.